1. Field of the Invention
The invention relates to a LIDAR method for measuring speeds, and to a LIDAR apparatus for carrying out a LIDAR method such as this.
2. Background Information
LIDAR is short for “light detection and ranging” and is a method, which is close related to radar (“radio-wave detection and ranging”) for range and speed measurement, as well as for remote measurement of atmospheric parameters. However, laser beams are used instead of radio waves, as in the case of radar.
One example of a Doppler LIDAR apparatus and of a method which can be carried out using it for measuring wind speeds is known from EP1756620 B1, US 20080117433 A1 or US 2006262324 A1.
In the case of direct reception Doppler LIDAR apparatuses, laser radiation is directed at the medium to be measured, and the radiation which is then reflected is received directly and is investigated for a Doppler shift in the laser wavelength, in order in this way to determine relative speeds.
Doppler LIDAR systems using direct reception technology are used, for example, for predictive measurement of turbulence, crosswinds or vortex trains in front of an aircraft, in particular an airplane. When carried on an aircraft, in particular an airplane, for measurement in front of the aircraft, which is considered in the following text to be a typical application, in addition to providing a pure warning function, the measurement signals can also in particular be injected directly into the aircraft flight control system in order, for example, to proactively regulate out gusts, crosswinds or vortex train influences, that is to say before the aircraft reacts negatively to the external flow change, thus maintaining a constant, smooth flight state, reducing loads on the aircraft, and ensuring safety in the aircraft and for the aircraft.
A number of major problems with previously known LIDAR systems which have been carried on an aircraft and has been designed for similar purposes will be mentioned, by way of example, in the following text:                Reflections back onto an optical window covering the LIDAR system, as a result of surface reflection or dirt, interfere with the measurement.        Background light, in particular background solar radiation in the optical range, must be suppressed, and this is complex.        It is desirable to optimize the measurement range such that the measurement is carried out sufficiently well in front of the aircraft, such that the measurement signals are available in the aircraft before flying through the disturbance. For example, the measurement signals should be injected into the flight control system at an early stage, such that the aircraft can be controlled to respond to the disturbance. However an adequate signal intensity, which decreases as the measurement range increases, should nevertheless be ensured overall for given accuracy requirements. This is not possible with previously known systems.        In previously known systems, the measurement depth cannot be selected or optimized sufficiently flexibly. However, optimization of the measurement depth is desirable such that the measurement can detect the turbulence gradients with sufficient resolution (turbulence gradients, turbulence length and turbulence coherence), while nevertheless overall ensuring an adequate signal intensity, which likewise decreases as the measurement depth decreases, for given accuracy requirements.        